This invention relates to a nitride semiconductor device used for light emitting devices such as a light emitting diode (LED) and a laser diode (LD), light receiving devices such as a solar cell and a light sensor and electronic devices such as a transistor and a power device, especially relates to an improved quantum well structure light emitting diode having an emitting peak wave length in a range of 450 to 540 nm wherein a loared operating voltage and an increased output can be obtained.
Nitride semiconductors have been used to make high bright and pure green and blue LEDs for full color displays, traffic signals and light sources for image scanner and so on. These LED devices are made of laminated structures which basically comprise a GaN buffer layer formed on a sapphire substrate, a n-type GaN contact layer doped with Si, an single-quantum-well (SQW) or multi-quantum-well (MQW) active layer comprising InGaN, a p-type AlGaN clad layer doped with Mg and a p-type GaN contact layer doped with Mg. The SQW blue laser having a peak wave length of 470 nm has shown a very superior characteristic such as the output of 2.5 mW and the external quantum efficiency of 5% at 20 mA, whereas the MQW has shown the output of 5 mW and the external quantum efficiency of 9.1% at 20 mW. Further, the SQW blue, laser having a peak wave length of 520 nm has shown the output of 2.2 mW and the external quantum efficiency of 4.3% at 20 mA, whereas the MQW has shown the output of 3 mW and the external quantum efficiency of 6.3% at 20 mW.
The MQW are expected to get an improved device characteristic such as higher outputs as compared to the SQW because the MQW can emit the light efficiently at a small current due to a plurality of mini-band structures. As a typical LED device having the MQW active layer for getting a good efficiency and output, Japanese Patent Kokai Hei 10-135514 discloses a nitride semiconductor light emitting device which comprises a MQW light emitting layer comprising laminated structures of undoped GaN barrier layers and undoped InGaN quantum well layers between clad layers having a wider band gap than that of the barrier layer. However, in order to improve the output of the blue green LED having a longer peak wavelength, there is proposed the increased number of layers in the MQW structure. The forward voltage Vf becomes higher depending on the layer number of MQW, resulting in such problems as the higher forward voltage Vf and the lowered emitting output.